Chemical Reaction Chemical Reaction EngineeringEngineering

CHE594

Assoc. Prof. Dr. Wan Shabuddin Wan Ali, Assoc. Prof. Dr. Wan Shabuddin Wan Ali, AMNAMN

Office: Level 19, Tower 2 (S&T)Office: Level 19, Tower 2 (S&T)

Tel: 03-5543 6351Tel: 03-5543 6351

H/P: 019-3511182H/P: 019-3511182

Januari 2009Januari 2009

Chapter 1Chapter 1

Overview of Chemical Reaction Engineering

- Classification of Reactions

- Variables Affecting the Rate of Reaction

- Definition of Reaction Rate

- Speed of Chemical Reactions

- Examples and Problems

Chapter OneChapter OneOverview of Chemical Reaction Engineering

Every industrial chemical process is designed to produce economically a desired product from a variety of starting materials through a succession of treatment steps.

Figure 1.1 shows a typical situation.

To find what a reactor is able to do we need to know the kinetics, the contacting pattern and the performance equation.

We show this schematically in Fig. 1.2.

the performance equation; relate input to output for various kinetics and various contacting patterns:

This important, Because with this expression we can compare different designs and conditions, find which is best, and then scale up to larger units.

[1]

Classification of Reactions

There are many ways of classifying chemical reactions. In chemical reaction engineering probably the most

useful scheme is the breakdown according to the number and types of phases involved, the big division being between the homogeneous and heterogeneous systems.

A reaction is homogeneous if it takes place in one phase alone.

A reaction is heterogeneous if it requires the presence of at least two phases to proceed at the rate that it does.

Table 1.1 shows the classification of chemical reactions .

Variables Affecting the Rate of Reaction

In homogeneous systems the temperature, pressure, and composition are obvious variables.

In heterogeneous, the problem becomes more complex. Material may have to move from phase to phase during reaction; hence, the rate of mass transfer can become important.

The rate of a reaction can be expressed as the rate of disappearance of a reactant or as the rate of appearance of a product.

Consider reaction (species A)

A → B

Definition of Reaction Rate

rA = the rate of formation of species A per unit volume-rA = the rate of a disappearance of species A per unit

volumerB = the rate of formation of species B per unit volume

The reaction rate is the increase in molar concentration of a product of a reaction per unit time.

If the rate of change in number of moles of i component due to reaction is dNi,ldt, then the rate of reaction in its various forms is defined as follows. Based on unit volume of reacting fluid,

Based on unit mass of solid in fluid-solid systems,

[2]

[3]

Based on unit interfacial surface in two-fluid systems or based on unit surface of solid in gas-solid systems,

Based on unit volume of solid in gas-solid systems

Based on unit volume of reactor, if different from the rate based on unit volume of fluid,

[4]

[6]

[5]

In homogeneous systems the volume of fluid in the reactor is often identical to the volume of reactor. In such a case V and Vr are identical and Eqs. 2 and 6 are used interchangeably.

In heterogeneous systems all the above definitions of reaction rate are encountered, the definition used in any particular situation often being a matter of convenience.

From Eqs. 2 to 6 these intensive definitions of reaction rate are related by

[7]

Speed of Chemical Reactions

Some reactions occur very rapidly; others very, very slowly.

Figure 1.3 indicates the relative rates at which reactions occur. To give you an appreciation of the relative rates or relative values between what goes on in sewage treatment plants and in rocket engines, this is equivalent to; 1 sec to 3 yr.

With such a large ratio, of course the design of reactors will be quite different in these cases.

EXAMPLE 1.1 A rocket engine, Fig. El.l, burns a stoichiometric mixture of fuel (liquid hydrogen) in

oxidant (liquid oxygen). The combustion chamber is cylindrical, 75 cm long and 60 cm in diameter, and the combustion process produces 108 kg/s of exhaust gases. If combustion is complete, find the rate of reaction of hydrogen and of oxygen.

Solution:

To evaluate:

Reactor volume and volume in which reactiontakes place are identical. Thus,

H2O produced = 108 kg/s X (1 kmol/18 kg) = 6 kmol/s

Thus, H2 used = 6 kmol/s and O2 used = 3 kmol/s

Rate of reaction is:

For H2:

For O2:

A human being (75 kg) consumes about 6000 kJ of food per day. Assume that I the food is all glucose and that the overall reaction is

Find man's metabolic rate (the rate of living, loving, and laughing) in terms of moles of oxygen used per m3 of person per second.

Read EXAMPLE 1.2

WorkWork PROBLEMS: CHAPTER 1 PROBLEMS: CHAPTER 11.1. Municipal waste water treatment plant. Consider a municipal 1.1. Municipal waste water treatment plant. Consider a municipal water treatment plant for a small community (Fig. P1.1). Waste water, water treatment plant for a small community (Fig. P1.1). Waste water, 32 000 m3/day, flows through the treatment plant with a mean 32 000 m3/day, flows through the treatment plant with a mean residence time of 8 hr, air is bubbled through the tanks, and microbes residence time of 8 hr, air is bubbled through the tanks, and microbes in the tank attack and break down the organic materialin the tank attack and break down the organic material

A typical entering feed has a BOD (biological oxygen demand) of A typical entering feed has a BOD (biological oxygen demand) of 200 mg O200 mg O22/liter, while the effluent has a negligible BOD. Find the /liter, while the effluent has a negligible BOD. Find the

rate of reaction, or decrease in BOD in the treatment tanks.rate of reaction, or decrease in BOD in the treatment tanks.

1.3 1.3 FluidFluid catalytic catalytic crackerscrackers (FCC)(FCC)FCC reactors are among the largest processing units used in the FCC reactors are among the largest processing units used in the petroleum industry. Figure P1.3 shows an example of such units. A petroleum industry. Figure P1.3 shows an example of such units. A typical unit is 4-10 m ID and 10-20 m high and contains about 50 typical unit is 4-10 m ID and 10-20 m high and contains about 50 tonstons of porous catalyst (of porous catalyst (ρρ = 800 = 800 kgkg//mm33). It is fed about 38 000 ). It is fed about 38 000 barrelsbarrels of of crude oil per day (6000 crude oil per day (6000 mm33//dayday at a density at a density ρρ = 900 = 900 kgkg//mm33), and it ), and it cracks these long chain hydrocarbons into shorter molecules. cracks these long chain hydrocarbons into shorter molecules. To get an idea of the rate of reaction; lets suppose that the feed To get an idea of the rate of reaction; lets suppose that the feed consists of just Cconsists of just C2020 hydrocarbon: hydrocarbon:

If 60% of the vaporized feed is cracked in the unit, what is the rate If 60% of the vaporized feed is cracked in the unit, what is the rate of reaction, expressed as of reaction, expressed as – r– r'' ((molesmoles reacted/ reacted/kgkg cat. scat. s) and as ) and as r"' r"' (moles reacted/(moles reacted/mm33 cat. scat. s)?)?

Next in Chapter 2:Next in Chapter 2:

KINETICS OF HOMOGENEOUS REACTIONS

• Simple Reactor Types

• The Rate Equation• CONCENTRATION-DEPENDENT TERM OF A RATE EQUATION

- Single and Multiple Reactions

- Elementary and Nonelementary Reactions

- Molecularity and Order of Reaction

- Representation of an Elementary Reaction

- Representation of a Nonelementary Reaction

- Testing Kinetic Models

• TEMPERATURE-DEPENDENT TERM OF A RATE EQUATION

- Temperature Dependency from Arrhenius' Law

- Comparison of Theories with Arrhenius' Law

- Activation Energy and Temperature Dependency